Posing Overlay Model Over Model Causes Clipping
If the two cubes deform in different ways, they surely have different weights. Remember that every deformation is leaded by more than a bone, if a vertex is weighted to one bone only, it moves exactly with that bone, regardeless the value of its weight.

Probably you have the same weights relatively to the head bone, but different weights relatively to some others bones (maybe shoulders and/or arms). To check this, select a vertex and search in the "N" properties panel for the "vertex weights" tab.